A conventional UPS power supply charges and discharges by means of a Direct Current/Direct Current (DC/DC) converter, or discharges by multiplexing a BOOST circuit of a rectifier and charges by multiplexing a double-BUCK circuit of the rectifier, thereby supplying an output voltage to a load. However, in a case that the UPS power supply discharges by means of only the DC/DC converter, a large discharge power is required in order to satisfy the load requirement, thus the switching loss is significant. In addition, the DC/DC converter is bulky and costly, and device utilization and system power density of the entire UPS power supply is relatively low. In a case that the UPS power supply discharges by multiplexing the BOOST circuit of the rectifier, high efficiency operation cannot be ensured in both a mains supply mode and a battery supply mode. Further, there is a problem that a high demand is put on the bus capacitor during switching.